1. Field of the Invention
The present invention relates to a solid electrolytic capacitor using a conductive polymer as a solid electrolyte.
2. Description of the Background Art
In recent years, a capacitor for high frequency having a low impedance in the high frequency range, a small size, and a large capacity has been demanded in accordance with miniaturization and reduction of weight of electronic devices.
A mica capacitor, a film capacitor, a ceramic capacitor, and the like are used as a capacitor for high frequency. However, these capacitors are the type of capacitor that is not suitable for a large capacity.
On the other hand, a capacitor suitable for increasing capacity includes an aluminum electrolytic capacitor, a tantalum electrolytic capacitor, and the like. The aluminum electrolytic capacitor is capable of achieving a large capacity with low cost. However, there are problems in which, because an electrolytic liquid is used, the change with the passage of time of capacitor characteristics occurs due to evaporation of the electrolytic liquid, and impedance in the high frequency range is high.
The tantalum solid electrolytic capacitor is a capacitor with less capacity deterioration because solid manganese dioxide is used as an electrolyte. However, because the coating film of manganese dioxide is poor in self-repairing property, there is a weak point of a danger such as combustion in the case that the dielectric coating film gets damaged during power distribution.
Therefore, in recent years, in order to solve the above-described problems, it is proposed to use a conductive polymer which is superior in conductivity and is allowed to easily form a solid electrolytic layer as a solid electrolyte. According to this method, a solid electrolytic capacitor with low manufacturing cost, sufficient capacitance to be certainly obtained, no damage of a dielectric coating film, and less leakage current, as compared with the above-described solid electrolytic capacitor has been obtained. Here, a polymer obtained by polymerizing pyrrole, thiophene, furan, aniline, and the like is used as the conductive polymer.
However, also in the solid electrolytic capacitor in which such a conductive polymer is used as a solid electrolyte, a reduction of equivalent series resistance (ESR), a further reduction of leakage current (LC), and the like are demanded in order to improve reliability.
In Japanese Patent Laying-Open No. 2005-116777, in order to solve the problems, a method of forming a conductive polymer as a solid electrolytic layer using a electrolytic polymerizing liquid obtained by mixing a plurality of dopants having different characteristics is proposed.